Amino acids and their derivatives have been described as corrosion inhibitors in lubricating oil compositions. For example, Japanese Patent 02142760 discloses the use of aqueous iron (II) aspartate salts as corrosion inhibitors for cast iron and Japanese Patent 6041388 deals with the use of a blend of acylated aspartic acid with lecithin as an oil soluble corrosion inhibitor. Similarly, Italian Patent 73-30278 describes aspartic acid amides as corrosion inhibitors in aqueous metal working systems and PCT application WO 91/04952 describes a metal salt of an aminocarboxylic acid as a corrosion inhibitor for use in coatings. U.S. Pat. No. 4,321,062 discloses the use of phenyl aspartates as corrosion inhibitors for gasoline and U.S. Pat. No. 4,228,304 teaches the use of aspartic acid derivatives of cyclohexanecarboxylic acid as anti-rust and emulsifying agents.
It is also known in the art that amino acids and their derivatives exhibit anti-wear properties or anti-wear synergy. For example Russian Patents 777053, 810781, 924090, 960232 and 1242507 teach the use of tetrasodium N-alkyl sulfosuccinoylaspartates to reduce tool wear. French Patent 85-14665 discloses overbased alkaline earth salts of amino acids as anti-wear detergent additives for lubricating oil, while European Patent 86513 (U.S. Pat. No. 4,462,918) discloses a lubricating oil composition with anti-wear properties or anti-wear synergy which contains a dialkyl ester of aminosuccinic acid of the formula ##STR1## where R.sup.1 and R.sup.2 are hydrogen, a hydrocarbyl radical containing 1-30 carbon atoms or an acyl derivative of the hydrocarbyl radical containing 1-30 carbon atoms and R.sup.3, R.sup.4, R.sup.5, R.sup.6, and R.sup.7 are hydrogen or hydrocarbyl radicals containing 1-30 carbon atoms and a Group II metal dithiophosphate.
Further, lubricating oil compositions containing amino acid derivatives with combined corrosion resistance and anti-wear properties are known in the art. French Patent No. 85-14663 describes a microdispersion, in oil, of metal salts of amino acids, including dicarboxylic amino acids which exhibit both anti-rust; and anti-wear properties. European Patent Application 0434464A1 teaches a lubricating oil composition containing an ashless sulfur and/or phosphorus anti-wear agent and an aminosuccinate ester corrosion inhibitor of the formula ##STR2## where at least one of R.sup.1 or R.sup.2 is an acyl group derived from a saturated or unsaturated carboxylic acid of up to 30 carbon atoms and the other can be hydrogen, an alkyl group of 1-30 carbon atoms or an acyl group derived from a saturated or unsaturated carboxylic acid of up to 30 carbon atoms, R.sup.3, R.sup.4, and R.sup.5 are hydrogen or an alkyl of 1-4 carbon atoms, and R.sup.6 and R.sup.7 are alkyl groups of 1-30 carbon atoms.
N-acyl-N-hydrocarbonoxyalkyl aspartic acid ester compounds, which are not disclosed by the foregoing citations, are known in the art. For example, N-(3-carboxy-5-ethyl-1-oxononyl)-N-[3-(decyloxy)propyl]-,1,4-diethyl ester and N-(3-carboxy-5-ethyl-1-oxononyl)-N-]3-{(2-ethylhexyl)oxy}propyl]-,1,4-diet hyl ester (Chemical Abstracts Registration Nos. 65626-32-6 and 65626-29-1) (Grenzflaechenakt. Stoffe, 4th, Teil 1) have been reported to be useful as nonfoaming and lye-resistant wetting agents. Similarly, N-acetyl-, 4-[2-hydroxy-3-{(1-oxooctadecyl)oxy}propyl] ester (Chemical Abstracts Registration No. 103946-51-6) (Arm. Khim. Zh., 41, (10), 603-9, 1988) has been reported to be useful as a surfactant. It has now been found that certain of such compounds are useful as corrosion inhibitors in lubricating oils and, unexpectedly, they possess the important advantage of acting as demulsifiers, a property not possessed or foreshadowed by their closely related analogs disclosed in the prior art lubricating compositions.
The term "demulsifier" as used in the present specification is intended to describe those compounds capable of preventing or retarding the formation of emulsions or capable of breaking emulsions. Demulsibilty is an important property in lubricating systems because condensation of atmospheric moisture often occurs on internal engine surfaces, especially on the interior of steam turbines as a result of temperature differentials. The normal action of an engine or turbine can beat the condensed moisture into an emulsion with the lubricating oil. The resulting emulsion has a reduced lubricity and consequently a reduced life as compared to the non-emulsified lubricating oil composition. Some common demulsifers are described in U.S. Pat. No. 3,957,854. Typically they comprise derivatives of ethylene oxide, such as ethoxylated or polyethoxylated organic mono- , di-, and triamines, ethoxylated carboxylic acid amides, ethoxylated quaternary ammonium salts, polyoxyalkylene alcohols and their ethers and esters, block polymers based on glycols, polyglycols, diamines or polyamines reacted sequentially with ethylene oxide or substituted ethylene oxides, and the esters and ethers of the described block polymers.
Dinonylnapthalene sulfonic acid and its metal salts form another important class of demulsifiers.
Demulsifiers typically function by changing the surface properties of the oil causing the emulsified water to coalesce. For example, in a steam turbine, the demulsifying agent in the lubricating oil causes the water to coalesce and settle in the sump from which it can be drawn off. Separation with a good demulsifier often occurs within 10-15 minutes, even if the oils contain zinc dialkyl dithiophosphates which tend to stabilize water in oil emulsions.